The present invention relates generally to a system and method of induced electric fields and, more particularly, to a system that provides induced electric fields that interact with the brain.
Psychiatric conditions are predominantly treated with pharmaceutical agents. For example existing treatment approaches for depression in bipolar disorder and in major depressive disorder utilize primarily pharmacologic agents, such as selective serotonin reuptake inhibitors and other antidepressant drugs. These agents can be of limited efficacy and may have objectionable side effects.
Repetitive transcranial magnetic stimulation (rTMS) has been used with the goal of treating depression, (see, e.g., George et al., The Journal of Neuropsychiatry and Clinical Neurosciences, 8:373, 1996; Kolbinger et al., Human Psychopharmacology, 10:305, 1995), bipolar disease and other psychiatric conditions. The success of rTMS in the treatment of depression has been varied and has been described in a recent review as “often statistically significant [but] below the threshold of clinical usefulness” (see Wassermann E M, Lisanby S H: Therapeutic application of repetitive transcranial magnetic stimulation: a review. ClinNeurophysiol 2001; 112:1367-1377). Furthermore, rTMS treatment can be unpleasant, with some patients declining participation due to scalp pain induced by the apparatus (George M S, Nahas Z, Molloy M, Speer A M, Oliver N C, Li X B, Arana G W, Risch S C, Ballenger J C: A controlled trial of daily left prefrontal cortex TMS for treating depression. BiolPsychiatry 2000; 48:962-970). The rTMS treatment also carries a small risk of seizure (Wassermann E M: Risk and safety of repetitive transcranial magnetic stimulation: report and suggested guidelines from the International Workshop on the Safety of Repetitive Transcranial Magnetic Stimulation, Jun. 5-7, 1996. Electroencephalogr Clin Neurophysiol 1998; 108:1-16).
Alternative techniques have been described for the treatment of psychiatric disease using low field strength, high repetition rates, and uniform magnetic gradients (U.S. Pat. Nos. 7,033,312 and 6,572,528, and U.S. patent application Ser. No. 11/580,272). Each of these patents and patent applications is incorporated herein by reference in its entirety. Time-varying magnetic fields were used for the purpose of enhancing brain function and for treating various symptoms of depression, anxiety, affective disorders, bi-polar disorder, post-traumatic stress disorder, and obsessive compulsive disorder.
Magnetic fields have also been used in Magnetic Resonance Imaging (MRI) systems. These systems use a coil to generate a magnetic field in air to which a portion of a subject's body can be exposed for imaging. A typical MRI coil is a full coil having four elements, such as that depicted in FIG. 5. The desired magnetic field is typically produced in a region in the middle of the coil—a region that is approximately equidistant from all four elements. Using MRI coils for treatment has several disadvantages, however, as described below.
One significant limitation on the use of an MRI gradient coil is its physical size, as imposed by the system, by power concerns, and by the patient. First, the larger the gradient coil, the larger its inductance, and the more the required power to operate the coil. A big coil usually requires more expensive amplifiers, and may impose power switching requirements that cannot be addressed merely by coil design. The portion of the patient which will be imaged must fit inside the coil, which imposes a lower limit on size. This is a limitation on minimum inner diameter of the gradient coil.
Second, the MRI coil fits inside the magnet. The cost and the difficulty in engineering required to make magnet both increase with the increase of the inner diameter of an MRI magnet. A MRI magnet must be large enough to accommodate a patient and the gradient coil within its inner diameter. A typical inner diameter of an MRI magnet must be large enough (e.g., about 90 cm) to provide an adequate opening so that the patient can be located at or near the region where the coil produces the desired magnetic field. This places an upper limit on gradient coil outer diameter.
An MRI gradient coil assembly typically contains 6 elements of gradient coils, two each for the X, Y and Z magnetic field gradient directions. A gradient coil assembly also usually contains resistive shim coils, and cooling for the resistive heat generated by the different coils. All of these items must fit within the inner and outer diameter limits imposed on the coil assembly by the patient access and the magnet size and cost.
In MRI systems, there is a need to cool the resistive heating that is generated inside the coil during operation—most systems need to have water or liquid cooling, because the coil is tightly packed between the inner and outer size limitations. Second, there is magnetic force on the wires in the coil when they have current in them; this causes a net force, usually in the form of a torque that can cause the coil to move.
In MRI systems, the dynamic magnetic fields are reflected from the surrounding magnet and would interfere with the desired target magnetic gradient fields. To prevent this, each gradient coil {X, Y, Z} is designed as a pair of coils—an inner coil and an outer coil—with the outer coil providing an active shield that prevents the gradient magnetic field from reaching the main MRI magnet. Thus, the outer coil merely prevents the magnet from interfering with the field produced by the inner coil.
A gradient coil that only surrounds a patient's head can have a smaller inner diameter, and as a result, may require less power and less cooling. The standard configuration of an MRI coil (i.e., the full coil having four elements as shown in FIG. 5), however, requires a length of coil to extend below the imaging area, i.e., the head. Put another way, the head must be positioned in the middle of the coil. Therefore, the coil must be large enough to accommodate shoulders of the person to be treated. Moreover, typical small-diameter coils do not have a strong mechanical mounting as that of the body coil, and hence, have a greater risk of movement from torque. This poses risk of severe patient injury. Finally, even though the MRI coils that have a relatively small diameter require less power, they still require cooling systems.
There was an attempt to address the shoulder access problem by several designs proposed in the 1990s. These designs used only a “half-coil” design. In this case, the half-coil reduces the extent of the coil below the imaging spot by cutting the coil in half, resulting in reduced gradient field homogeneity but allowing full access to the head, without requiring the person's shoulders to be surrounded by the coil. Such MRI coils, however, had significant torque and they were not safe for patient use. Also, the reduced gradient field homogeneity was not adequate for imaging purposes. The various issues relating to the use of MRI half coils are discussed, for example, in U.S. Pat. No. 5,177,442 to Roemer (describing half coil as having torque (as described by Kondo)); U.S. Pat. No. 5,278,504 to Patrick (describing an asymmetric coil which is not a half coil, in order to eliminate torque); and U.S. Pat. No. 5,793,209 to Kondo (classifying certain coils as effective in imaging but having a torque problem, and certain other coils as effective in torque mitigation but having imaging problems).
Therefore, there is a need for improved apparatuses, systems and methods for treatment of brain using electro-magnetic radiation which overcomes the disadvantages and limitations of the prior MR apparatuses and systems discussed above.